Gypsum set accelerator

ABSTRACT

A gypsum set accelerator, produced by compression milling a mixture of ground gypsum rock and starch, and the addition thereof to an aqueous slurry of settable calcined gypsum such as in the forming of paper-covered gypsum wallboard.

This application is a division of our copending application, Ser. No.514,455, filed Oct. 15, 1974, now U.S. Pat. No. 3,920,465, which saidapplication is a continuation of application Ser. No. 358,191, filed May7, 1973, now U.S. Pat. No. 3,870,538, issued Mar. 11, 1975.

This invention relates to a novel gypsum-starch composition for additionto a calcined gypsum slurry to accelerate the setting of the gypsum, andis particularly directed to an improved method of making gypsumwallboard.

Calcined gypsum, mixed with a proper amount of water, will normally setin about 25 minutes to a hardness such that a one-fourth lb. Gilmoreneedle will no longer make any substantial indentation, referred to asinitial set, and in about 35 minutes to a hardness such that a 1 lb.Gilmore needle will no longer make any substantial indentation, referredto as final set.

Two of the most common uses of calcined gypsum are (1) in forming thecore of the well-known paper-covered gypsum wallboards and (2) informing the relatively thick wall surfacing material in a plasteredwall. In making gypsum wallboard it has been common practice to add aset accelerating agent in an amount sufficient to permit the wallboardcore to reach initial set in about 3 to 4 minutes, and to reach finalset in about 5 to 8 minutes. In preparing bags of calcined gypsum forjob site mixture with water for plastering walls it has been common toadd set retarders and set stabilizers, which include accelerators, suchthat when admixed with water and aggregate the final set of the plastermixture is reached in about an hour to 2 hours.

The gypsum setting reaction involves the reaction of calcium sulfatehemi-hydrate and water to form calcium sulfate dihydrate. A most commonform of accelerator for this reaction is calcium sulfate dihydrate.There are many forms of calcium sulfate dihydrate which can be used andmany combinations of ingredients with which it is. combined, when usedas a set accelerator, depending on which of several alternativecharacteristics are most important for a particular use.

In preparing bags of plaster for use on job sites for plastering walls,the calcined gypsum, which is the primary ingredient, is commonly fed tothe bagging machine while it is still very warm from the calciningprocess. If a calcium sulfate dihydrate is added at this time, also, itis subject to being partially calcined, that is partially dehydrated, bythe heat from the calcined gypsum, thus losing much of its acceleratingability. To prevent this calcining of the accelerator, it has beentaught in U.S. Pat. No. 2,078,199 that a general group of water-solublecarbohydrates known as sugar or a material intermediate between starchand sugar, such as soluble dextrin, can be ball-milled together with thecalcium sulfate dihydrate in a weight ratio of 5-25:100 and thiswater-soluble carbohydrate will function as a dehydration-preventingsubstance when the combination is used as an accelerator.

The present invention is directed to an improved accelerator for the setof gypsum wherein the primary novel characteristics have no relation tothe dehydration prevention characteristics described above. The novelaccelerator provides completely unexpected and marked advantages whenused in the aqueous slurry of calcined gypsum for forming the core ofpaper-covered gypsum wallboard.

A common accelerator presently used in the manufacture of gypsumwallboard is fresh finely ground gypsum block. This finely ground blockcombined with calcined gypsum at a weight ratio of about 0.525:100 willprovide setting times suitable for wallboard manufacture, however itmust be used substantially immediately after grinding, and, secondly,once the wallboard core has set, this ground block in effect dilutes thestrength providing elements of the core, or to state it another way, itreduces the effective core purity.

The accelerator of the present invention consists essentially of thecompression-milled product of a mixture of calcium sulfate dihydrate andstarch. In a preferred form, the novel accelerator retains its potencyfor many days and involves the usage of only about 0.175:100 parts ofcalcium sulfate dihydrate to calcined gypsum. There would also beinvolved the usage of 0.175:100 parts of starch to calcined gypsum. Thisstarch, however, makes it possible to omit the separate addition ofstarch to the wallboard core formulation, which is an essential part ofprior formulations, and functions through migration to the core to paperinterface, to bond the paper to the core in the finished product.Furthermore, the total amount of starch required for providing bond ofpaper to core is less in accordance with the invention than with priorformulations.

It is an object of the invention to provide a novel accelerator for thesetting reaction of calcined gypsum and water.

It is a further object of the invention to provide a novel method ofmaking a gypsum accelerator and of making gypsum wallboard therewith.

It is a still further object of the invention to provide a novelwallboard formulation of substantially lower cost and improved quality.

These and other objects of the invention will be more readily apparentwhen considered in relation to the preferred embodiments as set forth inthe specification and shown in the drawings in which there is a flowchart depicting the process of the present invention.

In accordance with the invention, CaSO₄.2H₂ O, preferably in the form ofcrushed natural gypsum rock, is mixed with starch, preferably aninexpensive corn starch, and the mixture is subjected to the action of aball mill for a time sufficient to cause what appears to be a mechanicalcombining of the two materials and sufficient to produce a finalparticle size of about 10,000 to 20,000 cm² /gm or more particularlyfrom about 12,000 to 18,000 cm² /gm, and preferably about 15,000 cm²/gm, as measured by a Fisher Sub-Sieve Sizer.

As shown in the drawing this ball-milled mixture is then mixed with theother ingredients of a gypsum wallboard core, including calcined gypsum,paper pulp, foaming agent and water, and the core is formed betweenfacing papers. Of particular significance, it will be noted that theball-milled mixture provides both the set accelerator and the core topaper binder in the formulation.

Calcium sulfate dihydrate has been known to be an accelerator, in itsoriginal form as mined and finely ground, known as land plaster, andalso after it has been calcined and then rehydrated as in block form.The more expensive rehydrated block has been preferred due to itsuniform shape and thus its ability to be uniformly ground into a fineaccelerator additive. Grinding gypsum rock with abrasive wheels does notproduce a uniform rate of feed and it is essential that the plan groundland plaster be used immediately after being ground to a fine powder.

In the present invention, land plaster can be ground to a fineness ofabout 93% through a 100 mesh screen and stored any convenient timeperiod, and then be uniformly fed to a continuous feed ball mill alongwith a continuous feed of starch which is obtained at a fineness ofabout 86% through a 100 mesh screen. Alternatively, the mixture can beball milled in batches. The mixture, in accordance with the invention,can consist of from about 1 to 99% calcium sulfate dihydrate and fromabout 99 to 1% starch, however the most unexpected benefits of theinvention are obtained with mixtures of about 25 to 75% land plaster andabout 75 to 25% starch and particularly a ratio of about 1:1. One of theprimary advantages gained by the novel accelerator lies in theunexpected economics provided, and the best economy is obtained by theuse of land plaster and raw corn starch as the raw materials.

Some examples of starch suitable with the invention are CPCInternational (Corn Industrial Division) acid modified corn starch,Lauhoff Grain Company Hi Bond corn starch, A. E. Staley corn starch,Krause Milling Company grain starch, and Grain Processing Corporationpearl starch. The forms of starch which will provide the benefits of theinvention include those starches which, when dispersed within a wet, setgypsum core, will migrate to the core to paper interface during dryingand provide a bond of paper to core.

In addition to the above method using a ball mill, a rod mill can beused, or any other apparatus wherein the mixed ingredients are subjectedto a repeated compression milling treatment.

Following is an example of a prior standard gypsum wallboard coreformulation and an example of a preferred embodiment of the presentinvention, in amounts suitable for making about 1000 sq. ft. of 1/2 inchthick paper-covered gypsum board.

    ______________________________________                                                                  PRESENT                                                              PRIOR    INVENTION                                           ______________________________________                                        Calcined Gypsum    1491    lbs.   1491  lbs.                                  Starch (added separately)                                                                        3.4     lbs.   0                                           Paper Pulp Fiber Reinforcement                                                                   11.9    lbs.   11.9  lbs.                                  Ground Gypsum Block Accelerator                                                                  7.8     lbs.   0                                           Novel Accelerator (1:1 ratio)                                                                    0              5.2   lbs.                                  Water of Hydration 140.6   lbs.   140.6 lbs.                                  Soap (foaming agent)                                                                             .3      lbs.   .3    lbs.                                     Core Total      1655    lbs.   1649  lbs.                                     Front and Back Paper                                                                          144     lbs.   144   lbs.                                     Board Weight    1799    lbs.   1793  lbs.                                  ______________________________________                                    

The above two formulas are given as examples both of which will producegood quality 1/2 inch wallboard of about 1800 lbs/M sq. ft. The 6 lbs.difference in weight is of no importance. These two comparativeformulas, of slightly different total weight were used to best show thedramatic differences in certain of the individual ingredients in twootherwise similar formulas. All ingredients were kept constant exceptthe starch and accelerator to demonstrate how the novel accelerator,made from 2.6 lbs. of acid modified corn starch and 2.6 lbs. of landplaster, can perform as well as 3.4 lbs. of acid modified corn starchand 7.8 lbs. of ground block added separately.

The "Prior" formula had an initial setting time of 3'35" and a finalsetting time of 7'35". The "Invention" formula had an initial settingtime of 3'25" and a final setting time of 7'10". The bond of paper tothe gypsum core was satisfactory in both examples.

In another example of the invention, 5.2 lbs. of a novel accelerator wasused having 5% acid modified corn starch and 95% land plaster in placeof the 5.2 lbs. of 1:1 ratio novel accelerator above. The setting timeswere satisfactory but a poor bond of paper to core was obtained. This iscorrected for by adding additional starch as a separate ingredient. Thusthe novel accelerator of this 5:95 ratio is a satisfactory acceleratorin an unexpectedly lower total content but it lacks certain of the otherunexpected advantages of the higher starch content accelerator.

In another example of the invention, 5.2 lbs. of a novel accelerator wasused made from 50% pearl starch and 50% land plaster, in theabove-listed formulation. Pearl starch is an unmodified raw starch. Thisformulation provided an initial setting time of 2'30" and a finalsetting time of 4'45", indicating that 5.2 lbs. of this novelaccelerator is more than is needed.

In the above formulas, the amount of water of hydration is set forth. Anadditional amount of water, on the order of 1000 lbs., which is used inmaking the slurry from which the core is formed, which additional amountis subsequently removed during drying of the wallboard, is not set forthin the formula. It should be realized, however, that with the lesssolids in the Invention formula than the "Prior" formula,proportionately less additional water, to be subsequently dried off, isrequired.

The starch content of 3.4 lbs. in the Prior formulation should berecognized as a relatively low example of prior formulations, the rangebeing generally from about 3 lbs. to about 12 lbs., depending on manyother factors in the formulation and process, and thus it will befurther realized that the 2.6 lbs. of starch which provides a suitablebond in the Invention formula is a marked advantage.

Having completed a detailed disclosure of the preferred embodiments ofour invention, so that others may practice the same we contemplate thatvariations may be made without departing from the essence of theinvention.

We claim:
 1. The method of making gypsum wallboard comprising the stepsof compression milling a mixture of from about 1:3 to about 3:1 parts byweight of calcium sulfate dihydrate and starch for a period of timesufficient to reduce the particle size to provide a surface area ofabout 10,000 to about 20,000 cm² /gm as measured by a Fisher Sub-SieveSizer, mixing said milled mixture into an aqueous slurry comprising thewallboard forming ingredients including a major portion of calcinedgypsum and a minor portion water of hydration, fiber reinforcement anddensity reducing agent, said slurry also including excess water wherebythe slurry can be suitably formed, said milled mixture being onlyseparate additive for accelerating the setting and for providing thenecessary additional bonding strength needed by the settable gypsum tobond itself to paper cover sheets, disposing said slurry onto papercover sheets, forming said wallboards of said slurry and paper coversheets, maintaining said wallboard form while said slurry sets andhardens, and drying said wallboard.
 2. The method of claim 1 whereinabout 5 lbs. of said milled mixture of about 25 to 75% land plaster andabout 25 to 75% starch is added to said aqueous slurry per 1650 lbs. ofwallboard core produced.